Water treatment technologies including water desalination and purification are critical for addressing the issues of clean water shortage around the world1,2. Reverse osmosis (RO)3,4 and thermal processes5 are widely implemented in large-scale, industrialized desalination plants. However, large-scale plants consume a large amount of energy and involve high operating costs associated with infrastructure and skilled labor6, making them difficult to be implemented in developing countries and resource-limited areas. Smaller point-of-use (POU) potable water purification devices7-10, on the other hand, can avoid many of these obstacles and are increasingly recognized as one of the appropriate approaches to meet the needs of clean water and sanitation at the household and community levels.
POU water purification systems often comprise materials that adsorb contaminants, the most common being activated carbons obtained by a variety of methods11. However, while activated carbons can effectively remove organic contaminants and heavy metals, their capacity to adsorb salts is limited, and there are currently only few techniques that can efficiently desalinate water at a small scale3. Development of materials with high salt adsorption capacity will enable the realization of simple POU systems for direct desalination and purification of brackish water. Recently, carbon nanotubes (CNTs) have emerged as promising nanomaterials in water purification and desalination devices, mainly owing to three advantageous features12-15: (i) fast water flux enabled by the hydrophobic and frictionless graphitic walls; (ii) large surface area arising from the one-dimensional, high-aspect-ratio tubular structure; and (iii) ease of incorporating different functionalities on the graphitic walls of CNTs. It has been demonstrated both experimentally and theoretically that water permeability through CNT interiors could be at least three orders of magnitude higher than that predicted by the Hagen-Poiseuille law12,13. This high water flux could thereby significantly reduce the energy consumption in water treatments.
The most intriguing way of integrating CNTs into water purification and desalination devices is to use vertically aligned CNTs with functionalized open ends16,17. This aligned hollow structure allows pure water to pass through the inner core space of the nanotubes but reject salt ions and/or large-sized contaminants18,19. However, challenges with this approach include complex fabrication, scalability and clogging issues, as well as major difficulties in obtaining uniform CNTs with diameters smaller than the sizes of the solvated ions20,21. An alternative approach is to directly deposit CNTs onto a porous membrane and remove salt via capacitive electrostatic interactions22-24. Nevertheless, the salt rejection capability of CNTs in such configurations is poor for solutions with a high salt concentration due to the small Debye (electrostatic screening) length25,26. Other methods, such as surface functionalization with carboxylic and alkoxysilane-based chemical groups, were also reported to enhance water flux and desalination efficiency of such CNT-based membranes, e.g., as demonstrated in the case of membrane distillation27.
Hence, there exist a need for an improved membrane for water desalination and purification.